The miniaturization of wireless communications apparatuses such as mobile phones, etc. has resulted in rapid miniaturization of electronic devices used therein. FIG. 25 shows a high-frequency circuit for wireless communications apparatuses for wireless LAN (local area network). This high-frequency circuit comprises a high-frequency switch SW connected to an antenna ANT for switching connection to a transmission circuit Tx and a receiving circuit Rx; a filter FIL2, an amplifier PA, a filter FIL1 and a balun BAL1 being connected in this order from the antenna ANT in a pass for transmission signals of a frequency f1, and a filter FIL4, a low-noise amplifier LNA, a filter FIL3 and a balun BAL2 being connected in this order from the antenna ANT in a pass for reception signals of a frequency f2. Used in the transmission circuit Tx and the receiving circuit Rx in such high-frequency circuit are semiconductor devices IC handling balanced signals highly resistant to noises. Also, because the input/output impedance of a semiconductor device IC may be different from those of other circuit devices, baluns are sometimes needed for conversion between balanced signals and unbalanced signals, and for impedance conversion.
JP 2003-018039 A discloses an RF module comprising a switch circuit disposed between a transmission circuit and a receiving circuit and an antenna for controlling connection between the transmission circuit and the antenna and connection between the receiving circuit and the antenna, and a first balanced-unbalanced circuit disposed between the switch circuit and the transmission circuit; the first balanced-unbalanced circuit being a balun transformer comprising a first transmission line, and second and third transmission lines electromagnetically coupled to the first transmission line; the first transmission line having one end connected to an unbalanced terminal, and the other end which is grounded or open; the second transmission line having one end grounded, and the other end connected to the first balanced terminal; the third transmission line having one end grounded, the other end connected to the second balanced terminal; and a balun transformer constituting the first balanced-unbalanced circuit having second and third transmission lines whose ends are connected and grounded via a capacitor. Transmission lines and capacitors constituting the filters, the balun transformer and the switch circuit are formed on dielectric layers constituting a laminate, and passive devices not contained in the laminate, and active devices such as diodes, GaAs FET, RFIC, etc. are mounted on an upper surface of the laminate.
JP 2003-258585 A discloses, as shown in FIG. 26, a laminate-type electronic device comprising a filter and a balun contained in a laminate of insulation layers 310A-310J having conductor patterns 330-390, the filter and the balun being laterally displaced to avoid overlapping, and connected between an unbalanced terminal and a pair of balanced terminals.
The production of a small electronic device having pluralities of circuit functions with a laminate suffers some problems. The first problem is that because electronic devices having various circuit functions cannot be used in common applications, high-frequency circuits for which they can be used are restricted. For example, when various circuit blocks described in JP 2003-018039 A are produced, electronic devices adapted to them are needed, making it difficult to provide commonly usable electronic devices. The costs of industrial products generally decrease as their numbers increase, but conventional electronic devices cannot be produced at low cost because they are not usable for common applications. Also, once circuit specifications are changed, electronic devices adapted to them should be redesigned from scratch.
The second problem is the deterioration of characteristics by electromagnetic interference between pluralities of circuits. In JP 2003-258585 A, conductor patterns constituting a bandpass filter and those constituting a balun are formed on insulation layers with lateral displacement, to avoid overlapping between the bandpass filter and the balun in a lamination direction. However, the size reduction of electronic devices makes a bandpass filter closer to a balun in the laminate, making it difficult to arrange them so separately as to avoid interference.